YouTube user NewsPowerTV posted the three-minute video, which it said was "anonymously submitted" on Monday. It said the video showed officers "intimidating and threatening physical violence against protesters arriving in Chicago on May 9th, 2012". The user later posted a 30-minute, unedited, version of the encounter.

In the footage an officer is heard asking: "You guys got something planned for next week?" Before one of the protesters says they are heading to Occupy Chicago.

As the video continues, at the 12-minute mark a man's voice, apparently a police officer, says: "You like that, he knows, see these guys know, '68 … you guys all know '68", in an apparent reference to riots in Chicago during that year's Democratic National Convention, which resulted in more than 500 arrests.

The same voice continues: "What did they say back in '68?"

"A billy club to the fucking skull," another voice responds.

One of the protesters contends that the riots were related to "a race issue", prompting a voice to respond: "Ok, now we'll beat your white ass."

Later in the video, at around the 27-minute mark, an officer says: "Wait for the protest day. Save it up for then."

"We'll see you on Nato," a protester says.

A police officer apparently responds: "We'll come looking for you. Each and every one of you."

Local TV station WLS quoted Chicago police's superintendent Garry McCarthy as having said the video was "very clearly edited", although the posting of the unedited version – which came after McCarthy's comments – contains the same quotes as the original.

"That is not the full context of the conversation and less than encounter, except for that one comment, something about 1968 and night sticks and skulls, I think that those officers acted professionally, they acted courteously, and it was a casual encounter," McCarthy said.

"So I don't know the context of what was said, that the officer responded to that and I'm certainly not being defensive here. But let's be fair about it. If the officers did something wrong, then obviously we should take an action. That's not the right thing that an officer should say, and I'm not going to defend it. But let's get the whole picture here. Except for that comment that encounter was basically a non-event."

The Chicagoist news website said it had spoken with one of the people in the video, who said they had been pulled over by police while making a three-point turn. The protester said they had tried to record the conversation discreetly. The editing had been made to the earlier video to "exclude the downtime that occurred while police ran their IDs", Chicagoist reported.

In a statement on the incident, Chicago Police said: "During the course of patrol, police officers routinely conduct investigative stops if there is reasonable suspicion to do so. The individuals that were stopped were never handcuffed and were free to go after speaking with the officers."